As is known in the art, Electro-Magnetic Emission (EMI) compliance rules dictate the maximum amount of power a device is allowed to radiate at a particular frequency. In order to meet these requirements, a common technique employed is to spread the transmitted power over a frequency range in order to prevent the power from concentrating at a particular frequency.
FIG. 1 shows a conventional DC-DC converter 10 including a boost circuit 12 having an inductor 14, a diode 16 and a capacitor 18. A DC input voltage Vin is converted to a DC output voltage Vo, which can energize a load 20. A switching element 22 is coupled to the boost circuit 12 to determine the output voltage Vo. A control circuit 24 receives feedback from the load and uses this information to control operation of the switching element 22.
In fixed frequency DC-DC converters, power switching occurs at a constant frequency and results in radiated emissions concentrated at harmonics of the switching frequency, making it difficult to pass EMI requirements. In one known technique, in order to spread out the emitted power, the switching frequency is slightly changed in a random (or typically pseudo-random) fashion. Since the frequency is deviated by a small amount (e.g., +/−10%), the DC-DC converter maintains proper fixed-frequency operation and the emitted power is spread over a wider frequency span.